3 ms·

It took some digging, but I managed to find the specifications for the temperature limit switch in my High Efficiency heating system... for $7.62 I was able to

It took some digging, but I managed to find the specifications for the temperature limit switch in my High Efficiency heating system... for $7.62 I was able to buy a replacement from Grainger, and get it installed. I strongly suspect if I had called for service, I'd be told I needed a new furnace, even though this one is less than a decade old.

Brian_K_White 3y ago
Where was this limit switch? If it was on the flue, then it's a symptom of a drafting problem when they fail. Basically the thing should rarely ever click, but if the flu isn't drafting well, then the snap disk will get triggered frequently. You'll never notice anything wrong because the heat still works fine. Then the thing fails and you fix it, but you only fixed a symptom and not the problem. Even so, I still think it's better to get in there and figure stuff like that out than to avoid it. "just let the professional do it and do whatever they suggest" only works if you have some way of finding a guy who is both knowledgeable and honest. Yeah there are obviously risks like being unaware of invisible unsuspected things like above, but life is risk regardless.
wombatpm 3y ago
Check your filters. Some residential units have a mesh between the squirrel Cade fan and the fire box. Mine got partially block by an errant piece if insulation that was restricting air flow and causing the unit to short cycle. I too thought it was a bad switch. The HVAC dude said my condenser coil was clogged and I needed a new unit. One disassembly and cleaning later and it runs like new. When taking things apart take pictures showing where plugs wires and screws go.
toomuchtodo 3y ago
If it’s the high limit temp switch, that can be triggered by extended furnace runtimes when the temperature is so cold outside that the furnace must run nonstop to keep up with thermal losses (polar vortex and whatnot). This will potentially manifest in smart thermostats as frequent reboots, as the limit switch cuts off power to the furnace (which cuts power to the thermostat) to prevent it from bursting into flames. I can’t speak to Nest thermostats, but ecobee will send you an alert.
- mikewarot 3y ago
  It was the high temp limit switch, and it was tripping at about 110 degrees instead of 180. It kept short cycling, and the house got down to 58F. Brrrrr It took an extended run to get the house back up to temperature, but it had no problem with a correctly functioning switch.
- sokoloff 3y ago
  I took great pains to setup my boiler to run nearly continuously by using its outdoor reset controls and an outside temp sensor (a simple 10K NTC) to adjust the flow temperature to match/ever so slightly exceed what’s needed to match the building heat loss. This results in higher efficiency and better steady state comfort as the heat is on for always 18+ hours and often 22+ hours per day. It’s hard to say with confidence on the precision, but it seems like this is saving 8-10% in gas (in therms) vs last year when I had a constant flow temp (that I’d adjust manually every so often to try to match but where I’d have to leave it “too high” or risk a cold house on the coldest days). The exhaust temps in the stack reach steady-state far before even a 30 minute run would be finished.
- JesseHVAC 3y ago
  This is close, and I'd grant that limit trips are associated with longer run times. But in principle continuous furnace run time shouldn't cause a high limit trip. The limit is mainly to protect the heat exchanger from damage. The primary cause is usually low/reduced airflow, although bad sensors are also common. A high limit trip will generally cycle the furnace off for a time period. Somewhere between 3-5 off cycles will result in lockout and the furnace will stay off. The control for this is more likely to reside in the furnace board vs an aftermarket ui.
  - deleted 3y ago
    [deleted]
- deeesstoronto 3y ago
  The high temperature limit switch should never be tripping, even when the furnace runs continuously. This indicates you have a problem with temperature rise across the heat exchanger, probably from inadequate airflow. Your ducts may be sized too small/furnace too large, fan speed set too low, plugged filter, etc. You may be able to mitigate the problem by increasing fan speed, limiting heat by using stage 1 only, putting in less restrictive filter, add additional cold air return, etc. Usually a failed limit switch means cycling and a bigger problem with airflow. However I have seen a high temperature limit switch fail without cycling - fell apart with age (40 year old furnace).

Brian_K_White 3y ago

Where was this limit switch? If it was on the flue, then it's a symptom of a drafting problem when they fail. Basically the thing should rarely ever click, but if the flu isn't drafting well, then the snap disk will get triggered frequently. You'll never notice anything wrong because the heat still works fine. Then the thing fails and you fix it, but you only fixed a symptom and not the problem. Even so, I still think it's better to get in there and figure stuff like that out than to avoid it. "just let the professional do it and do whatever they suggest" only works if you have some way of finding a guy who is both knowledgeable and honest. Yeah there are obviously risks like being unaware of invisible unsuspected things like above, but life is risk regardless.

wombatpm 3y ago

Check your filters. Some residential units have a mesh between the squirrel Cade fan and the fire box. Mine got partially block by an errant piece if insulation that was restricting air flow and causing the unit to short cycle. I too thought it was a bad switch. The HVAC dude said my condenser coil was clogged and I needed a new unit. One disassembly and cleaning later and it runs like new. When taking things apart take pictures showing where plugs wires and screws go.

toomuchtodo 3y ago

If it’s the high limit temp switch, that can be triggered by extended furnace runtimes when the temperature is so cold outside that the furnace must run nonstop to keep up with thermal losses (polar vortex and whatnot). This will potentially manifest in smart thermostats as frequent reboots, as the limit switch cuts off power to the furnace (which cuts power to the thermostat) to prevent it from bursting into flames. I can’t speak to Nest thermostats, but ecobee will send you an alert.

mikewarot 3y ago
It was the high temp limit switch, and it was tripping at about 110 degrees instead of 180. It kept short cycling, and the house got down to 58F. Brrrrr It took an extended run to get the house back up to temperature, but it had no problem with a correctly functioning switch.
sokoloff 3y ago
I took great pains to setup my boiler to run nearly continuously by using its outdoor reset controls and an outside temp sensor (a simple 10K NTC) to adjust the flow temperature to match/ever so slightly exceed what’s needed to match the building heat loss. This results in higher efficiency and better steady state comfort as the heat is on for always 18+ hours and often 22+ hours per day. It’s hard to say with confidence on the precision, but it seems like this is saving 8-10% in gas (in therms) vs last year when I had a constant flow temp (that I’d adjust manually every so often to try to match but where I’d have to leave it “too high” or risk a cold house on the coldest days). The exhaust temps in the stack reach steady-state far before even a 30 minute run would be finished.
JesseHVAC 3y ago
This is close, and I'd grant that limit trips are associated with longer run times. But in principle continuous furnace run time shouldn't cause a high limit trip. The limit is mainly to protect the heat exchanger from damage. The primary cause is usually low/reduced airflow, although bad sensors are also common. A high limit trip will generally cycle the furnace off for a time period. Somewhere between 3-5 off cycles will result in lockout and the furnace will stay off. The control for this is more likely to reside in the furnace board vs an aftermarket ui.
- deleted 3y ago
  [deleted]
deeesstoronto 3y ago
The high temperature limit switch should never be tripping, even when the furnace runs continuously. This indicates you have a problem with temperature rise across the heat exchanger, probably from inadequate airflow. Your ducts may be sized too small/furnace too large, fan speed set too low, plugged filter, etc. You may be able to mitigate the problem by increasing fan speed, limiting heat by using stage 1 only, putting in less restrictive filter, add additional cold air return, etc. Usually a failed limit switch means cycling and a bigger problem with airflow. However I have seen a high temperature limit switch fail without cycling - fell apart with age (40 year old furnace).